In situ photolysis-electron spin resonance study of some reactions of phosphate radicals

Free radical intermediates formed in the reaction of phosphate radicals (PO42- or its protonated forms, HPO4- and H2PO4) with a number of fundamental organic and inorganic compounds have been studied by the in situ photolysis-steady state ESR method. The phosphate radicals were generated effectively by the photolysis of peroxodiphosphate (P2O84-) in aqueous solutions. Though the phosphate radical itself could not be detected directly, the ESR spectra of the phosphate radical adducts to fumaric and maleic acids and also the aci-anion of nitromethane were observed successfully. The pK/sub a/ value for the proton dissociation in the phosphate group of the adduct to fumaric and maleic acids, -OOCCH(OPO3H-)CHCOO-, has been determined to be 6.7. The reactions of phosphate radicals are rather similar to those of the related active species SO4-, e.g., they give rise to hydroxyalkyl radicals from aliphatic alcohols, adduct radicals with unsaturated compounds, and inorganic radicals such as CO3 from HCO3 and PO32- from HPO32-. However, there are notable differences in reactions with aliphatic and aromatic carboxylic acids. From saturated aliphatic mono- and dicarboxylic acids, α-carbon radicals produced by hydrogen abstraction with phosphate radicals were predominantly detected in neutral aqueous solutions and phenyl-type radicals were not detected from aromatic carboxylic acids. On the contrary the resultant radicals by decarboxylation were mainly observed in SO4- system. Clearly, direct oxidative electron transfer is not the predominant process in the reaction of the phosphate radical (HPO4-) with carboxylic acids